This past April the Camellia area of the Washington Park Arboretum was paid a scientific visit by UW SEFS professor Dr. Darlene Zabowski and students from her Advanced Soil Genesis and Classification course (SEFS 513). Their goal was to learn how to excavate a soil pit and mine the walls for information about the history of the site, the current state of the soil and potential issues that may need mitigation. The site was chosen by David Zuckerman, Supervisor of Horticulture, as our Camellia collection is in need of a renovation, and he’s a strong proponent of soil analysis prior to any work being done in an area.

As with any good assessment, photos of the site were taken prior to any disturbance:

This site is located in the south end of the Arboretum just north of the gravel path leading to the newly refurbished lookout in the New Zealand garden. After the leaf litter and duff were cleared, the students started digging, and digging until a 3 foot deep pit was completed (notice the clear separation of ‘horizons’, or layers of soil):

In this area 3 feet was needed to ensure that the students got down to the ‘parent material’, or the underlying geological material in which soil horizons form. Soils inherit structure and minerals from their parent material through processes of physical or chemical weathering. This parent material remains the basis of the soil structure as other factors contribute to the soil’s texture (e.g. compaction, amendments, tillage).  According to Dr. Zabowski and her students, our Camellia soil has a parent material in the ‘Alderwood series’, and it shows evidence of compaction and large quantities of amended materials in the upper horizon. There was charcoal found in the middle/upper horizons indicative of a fire in the area (perhaps post-logging) or the charcoal could have come in with amendments added to the soil years ago. The parent material is glacial, composed mostly of ablation till and basal till and the years of amending and alteration can be seen even down into these lower horizons.

As each horizon was unearthed, Dr. Zabowski (pictured above) had her students lay out a sample of the soil in ascending order to show and feel the difference from one layer to the next. The students were then charged with the task of coding out these samples by color using Munsell Soil Color Charts flip book. Soil color indicates the makeup of the soil within a given geographic area, which can influence the land’s fitness for usage. Samples of each horizon were also brought back to the lab and analyzed for chemical composition, bulk density, base saturation, and Cation exchange capacity (CEC). The Camellia soil was found to have a pH in the slightly acidic region (5.7-6.3), which is good for Camellias, as they like slightly acidic soil. The upper horizons of the soil were found to contain high levels of Ca, suggesting that there had been some CaCO3 added to the soil in the past (the high pH was also indicative of amending with CaCO3). The CEC of the soil was very high in the upper horizons, but this was to be expected at CEC is a measure of the soil’s fertility and nutrient retention capacity and this soil had been amended with organic matter for decades before this assessment (organic matter can have up to 3x the CEC of clay). The bulk density of the soil in the upper horizons suggests that there has been some compaction (likely due to foot traffic as there is a bench near the site) and that remediation of this density should accompany any work done in this area.

The UWBG horticulture staff welcomes and encourages university use of the arboretum for educational purposes as we curate and maintain 230 acres of urban forest as short walk from main campus. Got an idea for research in the arboretum? Contact David Zuckerman at dzman@uw.edu to get your shovels into our soil!